The present invention generally relates to implantable prostheses and the like which are formed in a manner to substantially prevent cracking, crazing or degradation thereof when they are implanted or otherwise subjected to degradation conditions. A medical prosthesis or the like according to this invention includes a polyolefinic elastomeric triblock star or linear copolymer where the backbone comprises alternating units of quaternary and secondary carbons which will not crack or degrade when subjected to implantation for substantial time periods during which other types of polymers would crack or degrade.
Several biocompatible materials which are quite suitable for use in making implantable medical devices that may be broadly characterized as implantable prostheses exhibit properties that are sought after in such devices, including one or more of exceptional biocompatibility, extrudability, moldability, good fiber forming properties, tensile strength, elasticity, durability and the like. However, many of these otherwise highly desirable materials exhibit a serious deficiency when implanted within the human body or otherwise subjected to harsh environments, such deficiency typically being manifested by the development of cracks or fissures. For example, surface fissuring or cracking occurs after exposure of on the order of one month or more, or shorter time periods depending upon the materials and the implant conditions, to body fluids and cells such as are encountered during in vivo implantation and use.
It is desirable that long-term implantable elastomers, such as those used for vascular grafts, endoluminal grafts, intraocular lenses, finger joints, indwelling catheters, pacemaker lead insulators, breast implants, heart valves, knee and hip joints, vertebral disks, meniscuses, tooth liners; plastic surgery implants, tissue expanders, drug release membranes, subcutaneous ports, injection septums, etc., be stable for the duration of the life span of the recipient.
Polymers that are not stable in the physiological environment tend to crack and degrade with time. There are many implant applications where this type of behavior cannot be tolerated. For example, pacemaker lead insulators can form current leaks thereby causing the wires to short out and the pacemaker to be rendered non-functional. It is therefore desirable to have a material for long-term use that is both elastomeric and does not degrade in the body.
Several theories have been promulgated in attempting to define the cause of this undesirable cracking phenomenon. Proposed mechanisms include oxidative degradation, hydrolytic instability, enzymatic destruction, thermal and mechanical failure, immunochemical mechanisms, inhibition of lipids and combinations of the above. Prior attempts to control surface fissuring or cracking upon implantation or the like have included incorporating antioxidants within a biocompatible polymer and subjecting the biocompatible polymer to various different annealing conditions, typically including attempting to remove stresses within the polymer by application of various heating and cooling conditions. Attempts such as these have been largely unsuccessful.
Other treatment approaches have been utilized, or attempted, to increase the structural stability of especially desirable materials. Included in the biocompatible materials which are desirable from many points of view, but which exhibit a marked tendency to crack or degrade over time, are the polyurethane materials and other biocompatible polymers that are of an elastomeric nature. It is particularly advantageous to use these types of materials for making products in which compliance and/or flexibility, high tensile strength and excellent fatigue life are desirable features. One basic approach which has been taken in the past in order to render these materials more suitable for implantation and other applications where material degradation can develop, has been to treat the material with so-called crack preventatives. Exemplary approaches in this regard are found in my U.S. Pat. Nos. 4,769,030, 4,851,009 and 4,882,148, the subject matter of which is incorporated by reference herein. Sulfonation of polyurethanes to prevent cracking is also described in my U.S. Pat. No. 4,882,148, the subject matter which is also incorporated by reference herein. Such treatments, of course, require additional procedures, and post processing of the implantable article, thereby complicating manufacturing procedures, increasing expense and complexity and, if not coated or treated properly and entirely, are subject to delamination and failure. It would be advantageous if the material out of which the product is made would itself have the desired properties. It is also advantageous for the material to be compatible with other materials that are commonly used in the medical fields, such as with adhesives, surface coatings and the like.
An especially difficult problem is experienced when attempting to form prostheses with procedures including the extrusion or spinning of polymeric fibers, such as are involved in winding fiber-forming polymers into porous vascular grafts or similar products, for example as described in U.S. Pat. No. 4,475,972 (Wong), the subject matter of which is also incorporated by reference herein. Such vascular grafts or the like include a plurality of strands that are of a somewhat fine diameter size such that, when cracking develops after implantation, this cracking often manifests itself in the form of complete severance of various strands of the device. Such strand severance cannot be tolerated to any substantial degree and still provide a device that can be successfully implanted or installed on a generally permanent basis whereby the device remains viable for a number of years.
There is accordingly a need for a material which will not experience surface fissuring or cracking under implanted or in vivo conditions and which is otherwise desirable and advantageous as a material for medical devices or prostheses that must successfully delay, if not eliminate, the cracking phenomenon even after implantation for months and years, in many cases a substantial number of years. Exemplary medical devices or prostheses for which such a non-cracking material would be especially advantageous include those which have been previously discussed.
The only elastomers that are currently implanted are polyurethanes, as previously discussed, and silicone rubbers.
The silicone rubbers, most notably polydimethylsiloxane, are probably the most stable elastomers used in the body. However, there have been many reported instances where they do not perform well. For example, silicone rubber poppet valves for coronary valve replacement tend to swell and crack with time, heart valve leaflets tend to calcify, silicone gel-filled breast implant shells tend to plasticize with silicone oils and in many instances, rupture with time. The mechanism of biodegradation of silicones in the body is believed to involve oxidative pathways.
Three families of polyurethanes have been used in long-term implant applications; i.e. the polyester urethanes which have been used as foamed coatings on some breast implants, the polyether urethanes which have been used as insulators on pacemaker leads, and the polycarbonate urethanes for use in vascular grafts. Polyether and polyester urethanes have repeatedly been shown to degrade with time in the body. L. Pinchuk, A Review of The Biostability and Carcinogenity of Polyurethanes in Medicine and the New Generation of "Biostable" Polyurethanes, J. Biomaterial Science, Polymer Ed., Vol 6, No. 3, pp 225-267 (1994).
The more recent family of biostable elastomeric polyurethanes which contain polycarbonate groups, rather than other or ester groups, are described in my U.S. Pat. Nos. 5,133,742 and 5,229,431, the subject matter of which is incorporated by reference herein. A similar polycarbonate urethane, but of a lower modulus of elasticity, is disclosed in U.S. Pat. No. 5,254,662 (Szycher et al). All of these polymers have demonstrated much improved biostability as compared to the polyether and polyester urethanes. However, as also described in my last mentioned review, some cracking and fiber breakage are observed on microfibers comprising a polycarbonate urethane vascular graft with time.
Still another biostable polyurethane is described in U.S. Pat. No. 4,873,308 (Coury et al). It is formed of all aliphatic soft segments of predominantly consecutive secondary carbon atoms. Two potential weaknesses of this polymer are that the secondary carbon atoms can oxidize with time, and the urethane linkages present on the backbone can hydrolyze with time. In addition, as reported in my aforementioned review, the polymer weakens with exposure to moisture and has a modulus with a yield point.
Again referring to my aforementioned review, a number of investigators have demonstrated that biodegradation of materials is usually a result of oxidation. Cells, especially leukocytes, secrete superoxide and hydrogen ions which subject the material to high concentrations of oxidants (free radicals) and strong acids. It is therefore a principal purpose of this invention to formulate long-term implantable materials with molecules that are not readily susceptible to oxidation and to attack by acid.
Two non-elastomeric polymers that have performed well in the body include polytetrafluoroethylene (Teflon) and polymethylmethacrylate with few, if any, reports of biodegradation. Other polymers in addition to those already discussed that have also enjoyed some measure of success in the body, but do demonstrate some degree of biodegradation with time, include polypropylene, polyethylene and to some degree polyester terephthalate (PET).
Examination of the chemistry of those polymers reveal that, except for polytetrafluoroethylene, which is inert due to the replacement of all hydrogens with fluorine, the most inert polymers are those with the most "quaternary" carbons, as defined below. The principal problem with all of these polymers is that they are non-elastomeric and, therefore, cannot be used in certain applications in the body, such as vascular or endoluminal grafts where elasticity is desired.
Polymethylmethacrylate has repeating units of: ##STR1##
Polypropylene has repeating units of: ##STR2##
Polyethylene has repeating units of: EQU --(CH.sub.2).sub.n --
The number of repeating units is usually sufficiently large so that the molecular weight of the polymer is in excess of 60,000 Daltons. It will be seen that polyethylene is comprised only of "secondary" carbons, i.e. each carbon atom on its backbone is bonded to two other carbon atoms. Polypropylene has alternating "secondary" and "tertiary" carbons. A "tertiary carbon" is a carbon that is bonded to three other carbon atoms. A "quaternary" carbon is a carbon that is bonded to four other carbon atoms. The polymethylmethacrylate has a backbone of alternating "quaternary" carbons and "secondary" carbons.
Further examination of polyethylene will reveal that, in the presence of free radicals and other oxidizing agents, the polyethylene molecule and its secondary carbons can undergo abstraction of hydrogens and the formulation of free radicals and double bonds, e.g. ##STR3## Double bonds can also lead to intermolecular or intramolecular crosslinking. Once the double bond, unsaturation or crosslinking forms in the polymer, the polymer can become embrittled leading to cracking or degradation. For this reason polyethylene is hardly used anymore for pacemaker lead insulators, because it embrittles and then cracks and flakes with flexion in the body.
Similarly, but not as frequently, polypropylene can oxidize to the formation of a double bond between the tertiary carbon and the secondary carbon, i.e. ##STR4## Polypropylene, when loaded with antioxidants, is successfully used as a suture in the body, but does show some degradation with time as a haptic on intraocular lenses.
On the other hand, polymethylmethacrylate, has quaternary and secondary carbons along its backbone. Therefore, it is not readily susceptible to oxidation. Formation of a double bond along the backbone of the polymer would require the cleavage of carbon to carbon bonds, i.e. ##STR5## rather than carbon to hydrogen bonds as in the secondary and tertiary carbons. Extremely high energies are required to break carbon to carbon bonds. It is for this reason that polymers with alternating quaternary and secondary carbon bonds are very stable in the body.
The problem with the polymethylmethacrylate, polypropylene and polyethylene polymers is that they are not elastomers. They are rigid engineering plastics. Therefore, they cannot satisfy a need in the medical industry for a flexible polymer with excellent oxidation resistance, such as one that has alternating units of quaternary and secondary carbons.
The present invention achieves these objectives with a polymer which is a polyolefinic elastomer of a triblock star or linear copolymer backbone having alternating units of quaternary and secondary carbons. The polymer should have a resultant hardness which is between about Shore 20A-75D, and preferably between about Shore 40A and Shore 90A.
Accordingly, a general object of the present invention is to provide improved crack-resistant devices and products.
Another object of the present invention is to provide a polymeric material and products made therefrom which are particularly resistant to cracking and degradation, even under in vivo conditions.
Another object of the present invention is to provide an improved polyolefin material which can be spun through a spinnerette or extruded through and/or into suitable molding devices into products which exhibit superior crack-resistant properties, and/or which can be injection or compression molded, solvent castable, or solvent sprayable into such products.
Another object of the invention is to provide improved implantable devices and/or prostheses which exhibit an exceptional ability to prevent the formation of cracks and strand severance upon implantation for substantial time periods, such as those needed for generally permanent implantation procedures.
Another object of the present invention is to provide an improved vascular graft and the like that is made from spun fibers of polymer and that exhibits exceptional stability with respect to crack formation and strand severance development under in vivo conditions.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.